1. Field of the Invention
The present invention relates generally to circuits of integrated optoelectronic devices and integrated microwave devices, and more particularly to substrates for such circuits.
2. Description of the Related Art
Optoelectronics pertains to the interaction of electronic processes with optical processes. This interaction is typically associated with energy conversion between electrical signals and optical signals. Optoelectronic devices, e.g., lasers, photodetectors, optical modulators and optical switches, are those devices in which this interaction takes place. Optoelectronic devices have been combined with microwave devices (devices operating in the signal region from around one gigahertz to several hundred gigahertz) to form optoelectronic/microwave circuits, e.g., transmitters, receivers and switching networks, which can be effectively used in various applications, e.g., optical-fiber communication links and signal-distribution networks.
Integration of optoelectronic/microwave modules has typically followed two paths--monolithic and hybrid. Monolithic integration places all active and passive components on the same substrate. In addition to reducing the overall circuit size, this process reduces parasitic inductances and capacitances because it shortens the length of circuit interconnect structures. Monolithic circuits are typically formed with compound semiconductor families, e.g., gallium aluminum arsenide (GaAlAs) and indium phosphide (InP), that inherently facilitate the realization of high resistivity substrates ( e.g., 1.times.10.sup.8 ohm-centimeter) which reduce microwave losses and crosstalk in electrical interconnects.
In an exemplary monolithic structure, heterojunction bipolar transistor (HBT) technology and molecular-beam-epitaxy (MBE) are combined to realize a monolithic integrated p-i-n photodiode and transimpedance amplifier in the indium phosphide semiconductor family (Walden, R. H., et al., "Broadband Optoelectronic Integrated Receiver Front Ends", Optical Fiber Communication 1994 Technical Digest, Paper TuH4, p. 33).
Another exemplary monolithic integration was directed to an optoelectronic/microwave transmitter for wideband communication in the 1-4 GHz frequency range (Yap, Daniel, et al., "Wideband Impedance-Matched Integrated Optoelectronic Transmitter", Integrated Optoelectronics for Communication and Processing, SPIE Vol. 1582, pp. 215-222). The transmitter combines a GaAs/GaAlAs single-quantum-well (SQW) ridge-waveguide laser with a GaAs MESFET amplifier-driver. The circuit was fabricated on a semi-insulating GaAs substrate. After the active devices were isolated by an etching process, passive microwave components, e.g., metal-insulator-metal (MIM) capacitors, spiral inductors and bias tees, were fabricated directly on the substrate.
In contrast with monolithic integration, hybrid integration connects discrete devices with electrical interconnects. Although this approach generally results in larger circuit size and higher parasitics relative to monolithic circuits, semiconductor materials and fabrication processes can be independently selected to enhance the performance of each device. In addition, hybrid integrated circuits can be developed faster and with less cost than monolithic integrated circuits.
In both monolithic and hybrid integration, optical transmission members, e.g., optical fibers and optical waveguides, must be precisely aligned with optical ports of the optoelectronic devices. In the particular case of lasers, alignment tolerances of .about.1 micron are typically required to restrict transmission loss penalties to less than 1 db.
In one approach to optical alignment (Schlafer, John, et al., "Microwave Packaging of Optoelectronic Components", IEEE Transactions on Microwave Theory and Techniques, Vol. 38, No. 5, May, 1990, pp. 518-522), discrete chips of optoelectronic devices, e.g., photodiodes and lasers, are mounted on high-resistivity substrates, e.g., diamond, ceramic and metal-polymer. The substrates are carried by a housing and optical fibers are secured to the housing with adjustable mounting blocks. Optical alignment is achieved by manually adjusting the fiber position while monitoring an output signal of the optoelectronic device. Although this structure can achieve satisfactory optical alignment, it is not suited to production because of its high demands on equipment and labor.
A more production-oriented, alignment structure uses flip-chip, solder-bump technology and V-shaped, substrate grooves (Wale, Michael, J., et al., "Self-Aligned Flip-Chip Assembly of Photonic Devices with Electrical and Optical Connections", IEEE Transactions on Microwave Theory and Techniques, Vol. 38, No. 5, May, 1990, pp. 518-522). An array of wettable interconnect pads are precisely positioned on a substrate carrier and on an optoelectronic device with photolithographic techniques. The grooves are formed in the substrate by anisotropic etching. They are configured and arranged to receive and position the optical fibers in a predetermined spatial location relative to the interconnect pads. The optoelectronic device is flipped over and the solder bumps positioned between the metal interconnect pads of the device and the substrate. Subsequent heating of the solder bumps causes them to reflow and their surface tension pulls the opposing interconnect pads into alignment. In a variation of this alignment structure, the optical fiber is replaced by an optical waveguide which is fabricated in the substrate carrier (Jackson, K. P., et al., "A High-Density, Four Channel, OEIC Transceiver Module", Journal of Lightwave Technology, Vol. 12, No. 7, July 1994, pp. 1185-1190).
In these latter alignment structures, the substrate carrier essentially functions as an optical bench for alignment of optoelectronic devices with optical transmission members. Passive microwave components must be realized on independent substrates, i.e., as chips, and electrical interconnects are limited to lossy structures, e.g., bond wires and ribbons. Although acceptable for monolithic devices, such carriers are not suitable for combining low-cost hybrid integrated circuits and compact, thin-film passive microwave components into optoelectronic/microwave circuits.